The present invention relates to a light source apparatus having a plasma light source for emitting light based on a plasma excited by microwaves, and more particularly to a light source apparatus which can be made much smaller.
FIG. 1 of the accompanying drawings shows a light source apparatus.
As shown in FIG. 1, a light source apparatus, generally depicted at reference numeral 1, comprises a magnetron 2, a waveguide 3 connected perpendicularly to the magnetron 2, a coupler 6 for maintaining an impedance match between it and the waveguide 3 and a coaxial resonator 7 interconnected to the coupler 6.
FIGS. 1-5 are related art, that is, they do not constitute prior art but, nevertheless, are related to the present invention. The description of FIGS. 1-5 is helpful in understanding the advantages achieved by the present invention.
The magnetron 2 generates microwaves with a frequency of approximately 2450 MHz. The waveguide 3 is connected to the magnetron 2 at a distance L6 from an end of waveguide 3 and a distance L7 from coupler 6 in the direction perpendicular to the direction in which microwaves are generated, and transmits microwaves generated from the magnetron 2.
The waveguide 3 is in the form of a square cross section directed toward a direction perpendicular to the direction in which microwaves generated from the magnetron 2 are transmitted. The waveguide 3 comprises a first metal rod 4 inwardly extended from the outside and a microwave absorber 5, each of the first metal rod 4 and the microwave absorber 5 disposed at a proper position thereof. The waveguide 3 is electromagnetically interconnected to the coaxial resonator 7 by the coupler 6. The coupler 6 is in the form of wedge, one end thereof, which is connected to the magnetron 2, being formed as a microwave-reflection surface and the other end thereof being thinner than the thickness of the waveguide 3.
The first metal rod 4 is projected into the waveguide 3 and moved into and from the waveguide 3 so as to achieve an optimum load impedance match of the magnetron 2.
Microwave absorber 5 is disposed on each side surface of the waveguide 3 for enabling the magnetron 2 to be operated stably.
The coaxial resonator 7 comprises the wedge-like coupler 6 elongated from the waveguide 3, a second metal rod 8 inwardly extended from the coupler 6, a coupling portion composed of a cylindrical member 9, i.e., a hollow waveguide, an electrodeless lamp bulb 10 in contact with the cylindrical member 9, the electrodeless lamp bulb 10 which is in the form of a ball-like glass container attached to the tip end of the cylindrical member 9 for sealing therein a mercury, or the like, and a cap 11 having many through-holes, i.e., a mesh outer waveguide.
The coupler 6 is extended from the waveguide 3 and is narrow like a wedge as shown in FIG. 1. The cylindrical member 9 has a height L1 and the cap 11 has a height L2, the height L1 and the height L2 being adjusted in such a manner that an impedance, which is obtained when the coaxial resonator 7 is seen from the coupler 6, becomes considerably small. In this situation, the electrodeless lamp bulb 10 is disposed at the position of an antiresonance state so that a situation in which an electric field is largest can be realized. At that time, the height L1 is about 1/4 of the wavelength of microwaves. The height L2 is longer than the height L1, resulting in a leakage of a microwave electric power being highly suppressed.
FIG. 2 shows the state of a voltage V and a current I obtained in the above condition. From FIG. 3, it is easily understood that the impedance, which is obtained when the coaxial resonator 7 side is seen from the coupler 6, is made extremely small.
Returning to FIG. 1, in order to match the impedance of the coupler 6 with respect to the coaxial resonator 7 and a characteristic impedance of the waveguide 3, the wedge-like waveguide is interconnected at its wedge-like opening with a height L4 smaller than the height L3 of the waveguide 3 to the waveguide 3 as a step-down transformer.
The second metal rod 8 is a rod made of metal for adjusting a resonance frequency of the coaxial resonator 7, and is inserted into or withdrawn from the coupler 3 in order to match a resonance frequency of the coaxial resonator 7 with a predetermined frequency.
A material, e.g., mercury, etc., sealed into the electrodeless lamp bulb 10 has to be converted into a plasma, which is thereafter excited to emit light, and therefore it should lie in a strong electric field. To this end, the electrodeless lamp bulb 10 is disposed in the coaxial resonator 7 composed of the cap 11, i.e., the mesh outer waveguide, and the cylindrical member 9 at its position where an electric field is strong.
The electrodeless lamp bulb 10 is forced to be cooled by air 12 which is flowed through the inside of the cylindrical member 9, i.e., the hollow waveguide, as shown by a solid arrow 12 in FIG. 1.
In this manner, energy of microwaves generated from the microwave generation source such as the magnetron 2 is introduced into the waveguide 3. Since the waveguide 3 and the coaxial resonator 7 are interconnected electromagnetically, the impedance is converted into a low impedance and microwaves are supplied from the waveguide 3 to the coaxial resonator 7 to activate a plasma of the material, e.g., the mercury or the like, sealed into the electrodeless lamp bulb 10 disposed within the coaxial resonator 7, thereby emitting light.
Characteristics obtained on the basis of relationship between microwaves generated from the magnetron 2 and the coaxial resonator 7 will be described below with reference to the drawings.
FIG. 3 is a Smith chart showing measured results of impedance of microwaves generated from the magnetron 2 when the frequency of the microwave is variable. Frequency characteristics (frequencies in a continuous range of frequencies between 2340 MHz to 2540 MHz) of the coaxial resonator 7 are illustrated in FIG. 3 for individual frequencies. If a length L5 (see FIG. 1) of the waveguide 3 is varied, then a whole frequency characteristic of the coaxial resonator 7 is rotated about the center of the Smith chart. Specifically, the characteristic of the coaxial resonator 7 is changed by varying the whole length L5 (see FIG. 1) of the waveguide 3. In the example shown in FIG. 3, a wavelength of microwaves transmitted through the waveguide 3 is represented by .lambda.g, and the length L5 of the waveguide 3 is expressed as: EQU L5=.lambda.g/4+N.times..lambda.g/2 (1)
N being an integer.
From the example shown in FIG. 3, it is easily understood that an oscillation frequency is drawing a clockwise trajectory as the frequency increases.
FIG. 4 is a Smith chart similar to the Smith chart shown in FIG. 3, showing selected frequencies in a continuous range, and showing measured results of load characteristic seen from the magnetron 2 side. From the Smith chart of FIG. 4, it is easily understood that an oscillation frequency is shifted in the counter-clockwise direction and increased with the change of a load impedance. In FIG. 4, a region that is shifted near a reference plane from a center point of the reference plane is a region (UNSTABLE) which an operation becomes unstable at low or high oscillation frequency.
If the characteristic presented by the waveguide 3 side as shown in FIG. 3, the characteristic (characteristic often referred to as a "pulling factor") presented by the magnetron 2 side as shown in FIG. 4, and the load impedance are in combination, then a region in which an operation becomes stable can be deduced from the operation principle.
A manner in which the state of the stable operation is achieved by combining the characteristics shown in FIGS. 3 and 4 will be described below.
Initially, the above region (UNSTABLE) that is shifted near the reference plane from the center point of the reference plane as shown in FIG. 4 should be avoided because it is unavoidably placed in the unstable state (i.e., the region in which oscillation is disabled) at any oscillation frequency.
According to an ideal combination of the above characteristics, the characteristic (see FIG. 4) of the oscillation frequency of microwaves generated from the magnetron 2 is not changed, and the characteristic of the oscillation frequency on the coaxial resonator 7 side can be adjusted by the coaxial resonator 7 side in such a manner that the region is placed above the center point of the reference plane and placed near the reference plane. Concretely, the oscillation frequency has to draw a trajectory placed above the center position of the reference plane and extended along near the reference plane shown in FIG. 4 instead of the clockwise trajectory shown in FIG. 3.
In order to realize such ideal combination, in the above light source apparatus, the length of the waveguide 3 can be finely adjusted equivalently by inserting into the first metal rod 4, which is attached to the upper surface of the waveguide 3, into the waveguide 3 or withdrawing the same from the waveguide 3 shown in FIG. 1. Further, the microwave absorber 5 made of a material capable of absorbing microwaves is disposed on each side surface of the waveguide 3.
FIG. 5 shows an equivalent circuit used to explain the above-mentioned operation principle.
As shown in FIG. 5, the microwave (wavelength of microwaves transmitted through the waveguide 3 is represented by .lambda.g1) generated from the magnetron 2 is transmitted through the waveguide 3 with the length L5=(.lambda.g1/4+N.times..lambda.g1/2) and reached to the coupler 6, whereafter it is converted by a voltage-drop transformer (not shown) in the coupler 6 into a low impedance and propagated to the coaxial resonator 7. The length L1 of the coaxial path from the coaxial resonator 7 to the electrodeless lamp bulb 10 is equal to a length of L1=.lambda.g2/4 and is able to generate a high electric field at an open end. Specifically, the coupler 6, which is coaxial with the electrodeless lamp bulb 10, for generating a plasma is expressed by a circuit of capacitors connected in parallel and a circuit of a resistor and a capacitor connected in series.
However, the light source apparatus 1 interconnects the magnetron 2 and the coaxial resonator 7 by the waveguide 3 to transmit energy of microwaves therethrough. Specifically, a wavelength .lambda.g of TE.sub.10 -mode wave that is generally used by the waveguide 3 to transmit energy of microwaves is given by the following equation (2): EQU .lambda.g=.lambda./(1-(.lambda./2.multidot.a).sup.2).sup.l/2(2)
where .lambda. represents a free space wavelength (=c/f) of available frequency, .lambda.g a wavelength of the waveguide 3, "a" a lateral dimension (diameter) of the waveguide 3, "c" a velocity of light, i.e., 2.9979.times.10.sup.8 m/s and "f" an operation frequency.
If the operation frequency f=2450 MHz and the lateral dimension "a" of the waveguide 3=100 mm, then the wavelength .lambda.g becomes about 155 mm.
As described above, the length L5 from the magnetron 2 to the coaxial resonator 7 has to satisfy the above equation (1) expressed as L5=.lambda.g/4+N.times..lambda.g/2.
Furthermore, in order to interconnect the magnetron 2 and the waveguide 3, a short-circuit surface should be formed at the position of .lambda.g/4 in a direction opposite to the direction in which the microwaves are transmitted. Accordingly, a required length L5' is given by the following equation (3): ##EQU1##
Considering the dimensions of the magnetron 2 and the coaxial resonator 7, the required length L5' becomes shortest if N=1. The minimum length of the waveguide 3 is 155 mm which is one wavelength, and this length of the waveguide 3 hinders the light source apparatus 1 from being miniaturized.
The light source apparatus 1 uses the waveguide 3 through which excited microwaves are transmitted to emit light as described above. However, the waveguide 3 is too long, which unavoidably hinders the light source apparatus 1 from being miniaturized. Therefore, the light source apparatus 1 poses a problem that it should be made much smaller while a performance of magnetron 2 is being maintained.